For example, in a vehicle using electric power as energy for propulsion, it is common to insulate a DC power source set at high voltage (for example, 200 V) from a vehicle body as an ungrounded power source. A detection circuit using a flying capacitor is used to detect an insulation state or a ground fault with respect to a ground potential portion of such an ungrounded power source.
In this detection circuit, the flying capacitor is respectively charged in the amount of electric charge according to a power source voltage of a DC power source insulated from the ground potential portion, the amount of electric charge according to a ground fault resistor of a positive terminal side of the DC power source and the amount of electric charge according to a ground fault resistor of a negative terminal side of the DC power source while switching an internal switch. Then, the respective charging voltages are measured in the controller side connected to the detection circuit, the ground fault or the insulation state can be detected by calculating the ground fault resistor of the positive terminal side and the ground fault resistor of the negative terminal side based on the measured value.
Incidentally, it is desirable to use a small ceramic capacitor with high capacity as the flying capacitor in recent years. In the ceramic capacitor, it is known that capacitance of the ceramic capacitor varies greatly depending on a DC bias. Hence, it is proposed to construct a detection circuit in which a charging voltage of a ceramic capacitor at the time when a ground fault resistor has a value of an alarm level becomes equal to a charging voltage by the amount of electric charge according to a power source voltage of a DC power source in order to eliminate the influence of a change in the capacitance due to the DC bias (for example, Patent Literature 1).